1. Field of the Invention
This invention relates to semiconductor wafer fabrication, and more particularly to a system and method for selectively conditioning a surface of a polishing pad of a chemical-mechanical polishing (xe2x80x9cCMPxe2x80x9d) apparatus in order to achieve a desired surface profile of a semiconductor wafer subsequently subjected to CMP using the CMP apparatus.
2. Description of Related Art
During a wafer fabrication process, multiple integrated circuits are formed upon frontside surfaces of each of several semiconductor wafers processed as a group or lot. Each integrated circuit consists of electronic devices electrically coupled by conductive traces called interconnects. Interconnects are patterned from conductive layers formed on the surface of a semiconductor wafer. The ability to form stacked layers of interconnects has allowed more complex circuits to be implemented in and on relatively small surface areas of semiconductor substrates. The individual interconnect levels of multilevel interconnect structures are separated by layers of electrically insulating materials (i.e., interlevel dielectric layers).
As the number of interconnect levels is increased, the stacking of additional interconnect layers on top of one another tends to increase the elevational disparities in frontside surface topographies. Problems arise when attempting to form interconnects upon rugged frontside surface topographies. Abrupt elevational changes in the frontside surface topography of a semiconductor wafer typically occur at or near lateral edges of underlying patterned features, e.g., interconnects. The tendency of layers formed upon the surface topography of a semiconductor wafer to be thinner over such abrupt elevations changes (i.e., xe2x80x9cstepsxe2x80x9d) is referred to as the xe2x80x9cstep coveragexe2x80x9d problem. In additional to the step coverage problem, large and abrupt elevation disparities lead to depth of focus problems. Depth of focus problems become an issue during the lithographic process in which layers are patterned across a semiconductor topography. A major factor in the processing of integrated circuits with submicron device dimensions is the limited depth of focus of the optical steppers used to pattern circuit features. In order to obtain maximum resolutions, imaging surfaces must be fairly planar with a suitable elevational disparity less than about 0.5 microns. Accordingly, interlevel dielectric planarization techniques must be employed in order to make imaging surfaces substantially planar.
Chemical-mechanical polishing/planarization (xe2x80x9cCMPxe2x80x9d) is a popular method of planarizing the upper surface of a layer (e.g., a dielectric or conductive layer) formed upon the frontside surface of a semiconductor wafer. CMP combines chemical etching and mechanical buffing to remove raised features upon the frontside surface of the semiconductor wafer. FIGS. 1 and 2 will now be used to describe an exemplary CMP apparatus. FIG. 1 is a top plan view of the exemplary CMP apparatus 10, and FIG. 2 is a side elevation view of exemplary CMP apparatus 10. CMP apparatus 10 is representative of, for example, a model Auriga or CMP-V polisher made by SpeedFam International, Inc. (Chandler, Ariz.). CMP apparatus 10 includes a platen (i.e., rotatable table) 12, a polishing pad 14, a wafer carrier or xe2x80x9cchuckxe2x80x9d 16, and a slurry delivery system 18.
As shown in FIG. 2, polishing pad 14 may include two separate disk-shaped polishing pads 14a and 14b stacked vertically upon one another. An underside surface of a first polishing pad 14a may be attached (e.g., adhesively) to a substantially planar upper surface of platen 12. A second polishing pad 14b may be attached (e.g., adhesively) to an upper surface of polishing pad 14a. Polishing pad 14a may be made of, for example, a rigid, microporous polyurethane material (e.g., a model IC1000 polishing pad made by Rodel, Newark, Del.). Polishing pad 14b may be made of, for example, a polyurethane-impregnated polyester felt material (e.g., a model Suba IV polishing pad made by Rodel).
Polishing pads 14a and 14b have outer diameters xe2x80x9cO.D.xe2x80x9d, and may be stacked as shown in FIG. 2 such that their outer diameters are vertically aligned. As shown in FIG. 2, polishing pad 14b has a hole in the center, and accordingly has an inner diameter xe2x80x9cI.D.xe2x80x9d. Polishing pad 14b also has a center line xe2x80x9cCxe2x80x9d midway between outer diameter xe2x80x9cO.D.xe2x80x9d and inner diameter xe2x80x9cI.D.xe2x80x9d as shown in FIG. 2.
During operation of exemplary CMP apparatus 10, a semiconductor wafer 20 is placed within wafer chuck 16. Platen 12 is set into rotational motion about a rotational axis 22 normal to the substantially planar surface. Wafer chuck 16 is set into rotational motion about a rotational axis 24. A force xe2x80x9cFxe2x80x9d is applied between wafer chuck 16 and platen 12 as shown in FIG. 2, pressing a frontside surface of semiconductor wafer 20 against the rotating upper surface of polishing pad 14 (i.e., polishing pad 14b). Slurry delivery system 18 delivers a liquid slurry to polishing pad 14, saturating polishing pad 14 with the liquid slurry. The liquid slurry may contain, for example, abrasive particles and a mild etchant chemical which softens or catalyzes the exposed material at the frontside surface of semiconductor wafer 20. Elevationally extending portions of the frontside surface of semiconductor wafer 20 are removed by combined chemical softening of the exposed surface material and physical abrasion brought about by relative movement between polishing pad 14 and the frontside surface of semiconductor wafer 20.
When used to planarize a semiconductor wafer surface, CMP apparatus 10 has two important performance factors: (i) polishing removal rate, and (ii) resultant semiconductor wafer surface planarity or xe2x80x9cuniformityxe2x80x9d. A high polishing rate is desirable in order to maximize the number of wafers which may be planarized in a given amount of time. A high measure of resultant semiconductor wafer surface planarity or xe2x80x9cuniformityxe2x80x9d is also desirable in order to reduce the step coverage and depth of focus problems described above.
The polishing rate performance of CMP apparatus 10 becomes degraded as waste materials build up on the upper surface of polishing pad 14 (i.e., polishing pad 14b) during use. The waste materials smooth out the textured upper surface of the pad, reducing the effectiveness of polishing pad 14. In order to maintain the effectiveness of polishing pad 14, the upper surface of polishing pad 14 is typically renewed periodically using a conditioning operation.
FIG. 3 is a side elevation view of CMP apparatus 10 wherein polishing pad 14 is undergoing an exemplary conditioning operation. The conditioning operation employs a pad conditioner 26 having a substantially planar abrasive surface 28. Abrasive surface 28 may include abrasive particles (e.g., diamond particles) embedded therein. During conditioning, platen 12 is set into rotational motion about rotational axis 22, and pad conditioner 26 is set into rotational motion about a rotational axis 30 normal to substantially planar abrasive surface 28. Abrasive surface 28 of pad conditioner 26 is brought into contact with the upper surface of polishing pad 14 (i.e., polishing pad 14b). As a result, a portion of the upper surface of polishing pad 14 is abraded (i.e., removed), along with any waste materials built up on the upper surface of polishing pad 14.
The semiconductor wafer surface planarizing or xe2x80x9cuniformityxe2x80x9d performance of CMP apparatus 10 is dependent upon the planarity of the upper surface of polishing pad 14. Past efforts to assess the uniformity performance of CMP apparatus 10 following the conditioning of polishing pad 14 include using CMP apparatus 10 to polish a surface of one or more xe2x80x9cqualificationxe2x80x9d wafers. Thicknesses of one or more layers formed upon the surfaces of the qualification wafers are then measured at various locations about the surfaces in order to determine the surface planarities of the qualification wafers. Such testing is not only time consuming, it is also wasteful in terms of material. The latter is especially true if the qualification wafers have operational circuits formed thereupon and the surface planarities of the polished wafers are unacceptable.
FIG. 4 is a sectional view of a portion of exemplary CMP apparatus 10 as indicated in FIG. 1. Past efforts to assess the uniformity performance of CMP apparatus 10 following the conditioning of polishing pad 14 also include attempts to assess the planarity of the upper surface of polishing pad 14 by measuring and comparing height xe2x80x9ch1xe2x80x9d of polishing pad 14b above the upper surface of polishing pad 14a at inner diameter xe2x80x9cI.D.xe2x80x9d and height xe2x80x9ch2xe2x80x9d of polishing pad 14b above the upper surface of polishing pad 14a at outer diameter xe2x80x9cO.D.xe2x80x9d as shown in FIG. 4. Height xe2x80x9ch1xe2x80x9d of polishing pad 14b above the upper surface of polishing pad 14a may be easily measured using a micrometer (e.g., a dial gage).
On the other hand, measuring height xe2x80x9ch2xe2x80x9d of polishing pad 14b requires either: (i) separating a portion of polishing pad 14b from the upper surface of polishing pad 14a long enough to measure height xe2x80x9ch2xe2x80x9d, or (ii) removing a portion of polishing pad 14b (e.g., cutting a notch or hole in polishing pad 14b) about outer diameter xe2x80x9cO.D.xe2x80x9d in order to measure height xe2x80x9ch2xe2x80x9d. Separating polishing pad 14b from the upper surface of polishing pad 14a, as well as removing any portion of polishing pad 14b, may reduce the polishing performance of polishing pad 14, and is thus undesirable. The above measurement method also assumes that the upper surface of polishing pad 14a underlying polishing pad 14b remains planar, which may or may not be true.
It would thus be desirable to have a method for determining the surface planarizing or xe2x80x9cuniformityxe2x80x9d performance of a CMP apparatus, following conditioning of a polishing pad of the CMP apparatus, which does not include separating the polishing pad from an underlying surface or removing any portion of the polishing pad.
The problems outlined above are in large part solved by a system and method for selectively conditioning a surface of a polishing pad of a CMP apparatus in order to achieve a desired surface profile of a semiconductor wafer. The semiconductor wafer is subjected to a CMP operation using the CMP apparatus following the conditioning. A CMP apparatus is described including a polishing pad having an underside surface mechanically coupled to a substantially planar surface of a platen. The platen is rotatable about a rotational axis normal to the substantially planar surface. The CMP apparatus also includes a substantially planar abrasive surface rotatable about a rotational axis normal to the abrasive surface. The abrasive surface may include abrasive particles (e.g., diamond particles) embedded therein.
A method for conditioning an upper surface (i.e., a polishing surface) of the polishing pad includes selecting a region of the polishing surface extending between a first and second radial distances from the rotational axis of the platen. The xe2x80x9cselected regionxe2x80x9d, encircling the rotational axis of the platen and bounded by the first and second radial distances, may be the region in which CMP is performed.
An existing first xe2x80x9cradial profilexe2x80x9d of the polishing surface within the selected region is determined. The term xe2x80x9cradial profilexe2x80x9d is used to describe a profile along a radial emanating from the rotational axis of the platen. A radial profile of the polishing surface xe2x80x9cwithin the selected regionxe2x80x9d extends along a radial and between the first and second radial distances defining the selected region. The radial profile exists in a plane perpendicular to the substantially planar surface of the platen and containing the rotational axis of the platen.
The determining of the existing first radial profile of the polishing surface within the selected region may include: (i) measuring a first existing distance between the polishing surface and the surface of the platen at the first radial distance from the rotational axis of the platen, and (ii) measuring a second existing distance between the polishing surface and the surface of the platen at the second radial distance from the rotational axis of the platen. The determining may reveal the extent to which the radial profile of the polishing surface within the selected region is xe2x80x9cslanted upwardlyxe2x80x9d in a radial direction with respect to the substantially planar surface of the platen, xe2x80x9cflatxe2x80x9d with respect to the substantially planar surface of the platen, or xe2x80x9cslanted downwardlyxe2x80x9d in a radial direction with respect to the substantially planar surface of the platen.
A desired second radial profile of the polishing surface within the selected region may be chosen, including: (i) a first desired distance between the polishing surface and the surface of the platen at the first radial distance from the rotational axis of the platen, and (ii) a second desired distance between the polishing surface and the surface of the platen at the second radial distance from the rotational axis of the platen. The first and second desired distances may determine the extent to which the radial profile of the polishing surface within the selected region is desired to be xe2x80x9cslanted upwardlyxe2x80x9d in a radial direction with respect to the substantially planar surface of the platen, to be xe2x80x9cflatxe2x80x9d with respect to the substantially planar surface of the platen, or to be xe2x80x9cslanted downwardlyxe2x80x9d in a radial direction with respect to the substantially planar surface of the platen. A correspondence between the radial profile of the polishing surface and the surface profile of the semiconductor wafer following CMP is established herein, and may be used as a basis for choosing the desired second radial profile of the polishing surface.
Conditioning of the polishing surface may involve rotating the platen and the abrasive surface about their respective rotational axes. When, the abrasive surface and the polishing surface are in contact, the polishing surface is abraded. The abrasive surface may be positioned such that the rotational axis of the abrasive surface is parallel to and a third radial distance from the rotational axis of the platen. The third radial distance may be constrained to lie between the first and second radial distances such that the selected region of the polishing surface is conditioned.
The third radial distance between the rotational axes of the abrasive surface and the platen may be selected dependent upon the existing first and desired second radial profiles of the polishing surface such that the desired second radial profile of the polishing pad is achieved during the conditioning. For example, when a difference between the second existing distance and the second desired distance is greater than a difference between the first existing distance and the and first desired distance, the third radial distance may be made greater than a radial distance midway between the first and second radial distances. In this case, a larger rotating surface area of the abrasive surface is in contact with an outer radial portion of the selected region. As a result, more material is removed from the outer radial portion of the selected region than an inner radial portion of the selected region. The contact between the abrasive surface and the polishing surface may be continued until the desired second radial profile is achieved within the selected region.
When a difference between the second existing distance and the second desired distance is equal to a difference between the first existing distance and the first desired distance, the third radial distance may be made equal to the radial distance midway between the first and second radial distances. In this case, equal rotating surface areas of the abrasive surface contact the inner and outer portions of the selected region. As a result, equal amounts of material are removed from the inner and outer portions of the selected region, and the radial profile of the polishing surface within the selected region is not changed.
When a difference between the second existing distance and the second desired distance is less than a difference between the first existing distance and first desired distance, the third radial distance may be made less than the radial distance midway between the first and second radial distances. In this case, a larger rotating surface area of the abrasive surface is in contact with the inner radial portion of the selected region. As a result, more material is removed from the inner radial portion of the selected region than the outer portion. The contact between the abrasive surface and the polishing surface may be continued until the desired second radial profile is achieved within the selected region.
A third existing distance between the polishing surface and the surface of the platen at the radial distance midway between the first and second radial distances may also be measured used to determine if the polishing pad is eligible for conditioning. It has been empirically determined that conditioning of the polishing pad to achieve a desired radial profile is most effective when the third existing distance lies between the first and second existing distances. Conditioning to achieve a desired radial profile is least effective when the third existing distance is not between the first and second existing distances. In this case, the polishing pad should be replaced.
The present CMP apparatus may include a measurement system for measuring a distance between the polishing surface and the substantially planar surface of the platen. The measurement system may include a sensor connected to a measurement unit by a cable. The sensor may produce a signal dependent upon a distance between a sensing surface of the sensor and the substantially planar surface of the platen, wherein the sensing surface may be placed in contact with the polishing surface. The cable may transmit the signal produced by the sensor to the measurement unit. The measurement unit may include a display device for displaying the distance between the sensing surface and the substantially planar surface of the platen. Alternately, the measurement unit may include signal conditioning circuitry, and may produce an output signal (e.g., an electrical voltage or current) proportional to the distance between the sensing surface and the substantially planar surface of the platen. The measurement unit may produce the output signal at an output port configured for connecting to a device for measuring the signal (e.g., a voltmeter or ammeter).
The platen may be formed from an electrically conductive metal (e.g., stainless steel), and the polishing pad may be made of an electrically non-conductive material (e.g., a polyurethane material or a polyurethane-impregnated polyester felt material). In this case, the sensor may include a coil of wire, and the measurement system may inductively measure the distance between the sensing surface of the sensor and the substantially planar surface of the platen through the electrically non-conductive polishing pad. The measurement unit may produce an electrical voltage proportional to the distance between the sensing surface of the sensor and the substantially planar surface of the platen at an output port.
The polishing pad may include stacked first and second polishing pads, wherein an underside surface of the first polishing pad is mechanically coupled to the substantially planar surface of the platen, and wherein an underside surface of the second polishing pad is mechanically coupled to an upper surface of the first polishing pad. The second polishing pad may have a hole in a center portion, and may have an upper surface which extends between the first and second radial distances from the rotational axis of the platen. The upper surface of the second polishing pad may be the selected region of the polishing pad.